In recent years, substantial efforts have been expended in the coating and finishing industry to reduce the organic solvent content of coating materials such as paint in order to avoid adverse environmental effects created by the solvents. Even high solids coating compositions, having a solids content of up to 60% percent by volume, include liquid solvent components which can escape during handling, atomization or deposition on a substrate, thus creating environmental and health hazards.
It has previously been proposed in Cobb's U.S. Pat. No. 4,247,581 to reduce the solvent content in paint by mixing a liquid or gas-blowing agent into the paint to produce an easily-atomized, foamed solution just prior to the spray orifice of a dispenser. Rehman, et al. U.S. Pat. No. 4,630,774, improved on this concept by designing a foaming chamber and turbulence-inducing device into the dispenser to better control the formation of the foam prior to the spray orifice. U.S. Pat. Nos. 4,505,406; 4,505,957; and 4,527,712 also disclose concepts for intermixing liquid or gas-blowing agents into paint formulations to reduce solvents.
More recently, U.S. Pat. No. 4,923,720 to Lee, et al. disclosed a method and apparatus for the production of a coating formulation in which a substantial amount of the liquid solvent component is removed and replaced with a supercritical fluid such as supercritical carbon dioxide which functions as a diluent to enhance the application properties of the coating formulation. The supercritical carbon dioxide and some liquid solvent material, e.g. about two-thirds less than is required in other coating compositions, are intermixed with polymeric and pigmentary solids to form a coating material solution or formulation having a viscosity which facilitates atomization through an airless coating dispenser. As the coating material formulation is discharged from the dispensing device toward a substrate, the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating composition and to reduce drying time of the composition on the substrate. Such coating material formulation, like the earlier prior art, has the advantage of substantially reducing the adverse environmental effects caused by coating compositions having a high solvent content.
The Lee, et al. U.S. Pat. No. 4,923,720 discloses an apparatus in which a liquid coating composition and a supercritical fluid are supplied from separate sources to a mixer wherein the two components are combined to form a coating material solution or formulation which is delivered to one or more coating dispensers for deposition onto a substrate. In the embodiment of the system disclosed in the Lee, et al. patent, the liquid coating composition and supercritical fluid are each introduced into the system by a separate piston pump. These two piston pumps are slaved together by a shaft which extends between the pistons of the two pumps, and the shaft position is adjusted to control the length of the piston stroke in each pump. The length of each piston stroke, in turn, governs the volume of the liquid coating composition and the volume of the supercritical fluid entering the system.
A number of problems with the design described in U.S. Pat. No. 4,923,720 have been addressed in U.S. patent application Ser. No. 07/728,051 filed Jul. 15, 1991 entitled "Method and Apparatus for Forming and Dispensing Single and Multiple Phase Coating Material Containing Fluid Diluent" to Saidman, et al., which is owned by the assignee of this invention. As noted in that application, a principal limitation of the system of U.S. Pat. No. 4,923,720 is that control of the relative proportion of liquid coating composition to supercritical fluid is difficult. This is because adjustment of the volume of one material entering the system automatically produces an adjustment of the volume of the other material since the two piston pumps which introduce such materials are slaved together by a common shaft. No provision is made in the Lee, et al. system for adjustment of the volume of one material introduced into the system independently of the other, and/or to take into account such variables as pump cavitation, fluid viscosity, leakage of supercritical fluid, uneven mixing of the supercritical fluid and liquid coating composition and other variables.
The invention disclosed in patent application Ser. No. 07/728,051, now U.S. Pat. No. 5,215,253, overcomes many of these problems with a system in which supercritical fluid and liquid coating composition or resin are introduced from separate sources into a loop or continuously-circulating flow path wherein the two materials are combined to form a coating material solution or formulation which is then supplied to coating dispensers for deposition onto a substrate. In order to maintain the proper proportion of supercritical fluid and resin, a control system is provided which monitors a parameter of the formulation which can be correlated to either the fluid diluent content or the resin content of the formulation. In the preferred embodiment of Ser. No. 07/728,051, such control system includes a capacitor located downstream from a point at which the fluid diluent and resin are intermixed. This capacitor forms part of a capacitance bridge or capacitance sensing circuit which produces a signal corresponding to the dielectric constant of the mixture. This dielectric constant can be correlated to the fluid diluent content of the mixture, and, depending upon the capacitance sensed by the capacitor, a control valve is operated by a computer associated with the capacitance sensing circuit to adjust the flow of supercritical fluid supplied to the loop so that a predetermined proportion or ratio of supercritical fluid-to-resin is maintained.
One potential problem with systems of the type disclosed in Ser. No. 07/728,051 can occur in applications wherein a fluctuating demand for the formulation is required. For example, in some applications, a number of coating dispensers may be intermittently turned on or off, and/or dispensers may be operated independently of one another such that different numbers of dispensers are turned on while others are not operating. As noted above, the system of Ser. No. 07/728,051 maintains the proper ratio of supercritical fluid-to-resin by first intermixing these components within a circulating flow path or loop, sensing the capacitance of the mixture and then adjusting the quantity of supercritical fluid introduced into the loop dependent upon the sensed capacitance. But there may be instances where the demand for the formulation is so high, e.g. by suddenly turning on a large number of dispensers, that the mixture of supercritical fluid and resin cannot be circulated around the loop in order to sense its capacitance and adjust the input of supercritical fluid before the mixture must be discharged to the coating dispensers. As a result, the relative proportion of supercritical fluid-to-resin may not be maintained as precisely as desired during such periods of widely-fluctuating flow requirements.
Another potential problem with systems of the type disclosed in the Lee U.S. Pat. No. 4,923,720, and Ser. No. 07/728,051, is that color change operations are relatively time consuming and difficult. This is because in systems of this type, both the resin and supercritical fluid are introduced into a common circulating flow path or loop where they are combined prior to discharge to one or more coating dispensers. In order to change from a resin of one color to another, the entire loop must be flushed of the old resin, including each element within such loop, before a new colored resin can be introduced into the system. This operation can be time consuming and, hence, unacceptable for certain applications.
In addition to the foregoing, systems of the type disclosed in the Lee, et al. U.S. Pat. No. 4,923,720, and application Ser. No. 07/728,051 to Saidman, et al., are intended to form coating material formulations or mixtures comprising resin and supercritical fluid as a diluent. No provision is made for the addition of other components or for controlling the proportion of such other components in the overall mixture. In certain applications, such as, for example, when rapid drying time is advantageous, "two component" coating material formulations such as paints are desirable. Two component paints generally comprise a "high volume component" consisting of polymeric pigmentary solids and organic solvents, and a "low volume component" such as a catalyst. Depending upon the particular resin employed, the ratio of resin-to-catalyst can be on the order of up to 100 to 1, and the flow rate of the catalyst can be as small as 3 ccm/min. Not only do the Lee, et al. and Saidman, et al. systems fail to contemplate the addition of a third component within the circulation flow path or loop, but neither system is designed to accommodate the high ratios and low flow volume required for some two component paints.